This invention relates generally to fluorescent lighting devices, and in particular, relates to such devices capable of providing three levels of light when energized from a three-way incandescent lamp socket.
Fluorescent lamp devices replacing single wattage incandescent lamps are known and achieve substantial operating economy over the incandescent lamps they replace. They generally comprise a fluorescent lamp, usually arranged in a major portion of a circle, having preheatable cathode filaments in each end and a line voltage starter circuit connected across the two filaments to start the lamp. One end of the lamp is connected directly to the shell of a standard, screw-in, incandescent lamp base. The other end is connected to the base pin by a choke, such as a reactor or an inductor, which serves to limit or ballast the operating current of the negative resistance fluorescent lamp. The choke is constructed and arranged to pass sufficient current to the preheat filaments to insure proper lamp starting and limit the operating current to below the maximum lamp operating current. In actuality, the preheat filament current necessary to raise the filaments to a sufficiently high temperature for proper lamp starting is similar to or greater than the lamp operating current. This is generally for the fixed wattage replacement device operating at a fixed light level, which is equivalent to the light level produced by the replaced incandescent lamp.
Flurorescent lamp devices replacing multiple wattage, such as three-way, incandescent lamps also are known, but until now have failed to achieve the light levels produced by the replaced incandescent lamps, particularly in the "LOW" and "MEDIUM" (hereafter "MED") modes of operation. Popular incandescent three-way lamps have wattage ratings of 30/70/100 and 50/200/250, the numbers respectively indicating the LOW, MED and HIGH operating wattages. The light level produced in each mode is directly related to the wattage so that a 30/70/100 watt incandescent lamp produces light levels of respectively, approximately 30%, 70% and 100% of the maximum or HIGH light level.
A typical three-way fluorescent lamp device, exemplified by the devices described in U.S. Pat. Nos. 4,349,768 and 4,178,535 to Miller, connects one end of the fluorescent lamp to the shell of a standard, screw-in, three-way incandescent lamp base. The other end is connected by a first series choke of a high reactance value to the center pin of the base or plug and by a second, series choke of a low reactance value to the ring of the base. In the LOW mode, current flows through the pin, first choke, lamp and base shell. In the MED mode, current flows through the ring, second choke, lamp and plug. In the HIGH mode, current flows through the pin and ring, both chokes in parallel, the lamp and the base shell. The LOW and MED light levels thus are provided by the independent action of such dual first and second chokes, while the HIGH light level is provided by the parallel action of the first and second dual chokes.
The high reactance value of the first choke limits the fluorescent lamp operating current to a low level, which, in turn, determines the LOW light level produced by the lamp. The low reactance value of the second choke passes a higher lamp operating current, determining the MED level of light produced by the lamp. The parallel reactance of the first and second chokes passes a still higher current, determining the HIGH light level. The lamp operating current in the HIGH mode may be greater than the sum of the currents in the LOW and MED modes due to the negative resistance characteristics of the fluorescent lamp.
The known three-way fluorescent replacement lamp devices, however, have been unable to achieve LOW light levels less than about 50-60% of the HIGH light levels. The problem is inexpensively providing sufficient lamp preheat filament current for proper starting while limiting LOW mode operating current to a substantially lesser current. This problem has been skirted by limiting the LOW mode operating current to produce a LOW light level of about 45% of the HIGH level. The MED mode light level then is set at about 55% of the HIGH level, and by careful selection of the choke reactance values, proper starting is insured. The small percentage light level in the LOW mode, however, is not achieved.
Alternatively, the 50% limitation has been avoided by over driving the fluorescent lamp in the HIGH mode; providing for lower percentage LOW mode light levels, but reducing lamp life.
It is known to use a positive temperature coefficient (PTC) resistor or thermistor in a fluorescent lamp device replacing a three-way incandescent lamp, see U.S. Pat. No. 4,386,296. The device described there, however, produces only two light levels: LOW and HIGH. The circuit of that device uses only one choke, which passes current for both modes of operation and includes a thermistor which operates to produce the LOW mode light level. Essentially, the choke is connected between the base ring and the second end of the fluorescent lamp. The thermistor is connected between the base pin and the end of the choke connected to the base ring. When power first is applied in the LOW mode to the pin, the thermistor is of low resistance and passes sufficient current through the choke and preheat filaments to start the lamp. Shortly thereafter, the thermistor temperature, and therefrom, resistance increases to limit the operating current to the desired LOW light level. Two disadvantages of this device are the limited number of light levels and the use of the thermistor as a current limiting and energy wasting device. A waiting period must pass before the thermistor cools sufficiently to pass enough current to the preheat filaments for restarting the lamp in the LOW mode.
What is desired, then, is a fluorescent lamp device capable of providing the three light levels realized from a three-way incandescent lamp. In such a device, in the LOW and MED modes of operation, both sufficient preheat filament current must be available for proper starting, and the light level percentages should be similar to those of the comparable incandescent lamp. Further the device should exhibit little or no time delay in starting the fluorescent lamp in any mode.